1. Field of the Invention
Exemplary embodiments of the present invention generally relate to a transfer device and an image forming apparatus including the transfer device, and more particularly, to a transfer device including an endless belt to transfer a visible image formed on each of multiple image carriers onto either an endless belt member or a recording medium carried on a surface of the endless belt member to generate a composite toner image, and an image forming apparatus for forming images using the transfer device.
2. Discussion of the Related Art
Image forming apparatuses employing a tandem electrophotographic method are well known, and typically include multiple image carriers to form single-color toner images of different colorants thereon, and a transfer device to sequentially transfer these single-color toner images directly onto a recording medium carried on a belt member serving as a sheet conveying belt or via a belt member serving as an intermediate transfer belt before transferring to the recording medium, so as to form a full-color image in overlay.
In such a tandem electrophotographic image forming apparatus, a change in temperature of optical components such as lens and mirrors of a writing unit can cause fluctuations in optical paths of light, where even a slight fluctuation of optical paths can lead to relative displacement of write start positions between image carriers. When the writing unit starts writing respective latent images while the latent images are relatively out of registration, improperly formed single-color toner images are sequentially transferred onto the belt member or the recording medium, and as a result, a defective full-color image having a color shifted from its original image position is produced.
To correct the above-described color shift, the tandem electrophotographic image forming apparatus forms given toner images or pattern images for misregistration detection at the multiple image carriers that are then transferred onto a belt member in a single layer, detects relative misregistration of the single-color toner images based on a timing with which optical sensors detect the single-color toner images of different colorants in the pattern images, and adjusts writing start timing of the latent images to their respective image carriers as well as any skew or displacement of the optical components based on the above-described detection results. Accordingly, relative misregistration of the single-color toner images between the image carriers can be reduced or prevented.
However, there is another factor contributing to misregistration of the single-color toner images, which is fluctuation in drive speed of a belt member that can be caused by, for example, uneven thickness of the belt member in a direction of movement of the belt member, eccentricity of a drive roller driving the belt member, etc. While the relative displacement of the write start positions of the single-color toner images is related to the image carriers that provides images, the speed fluctuation of a belt member is related to the belt member that receives the images. Consequently, the speed fluctuation of a belt member during a transfer operation can result in misregistration between images, even though the relative positions between the single-color toner images on the image carriers are properly arranged.
More specifically, registration error of respective single-color toner images caused by fluctuation in the speed of the belt member cannot be eliminated by adjustment of the latent image write start timing and/or adjustment of inclination of the optical components. Since misregistration of single-color toner images caused by the relative displacement of the write start positions between the image carriers causes misregistration between the single-color toner images of different colorants, relative positions of dots of the respective single-color toner images in the composite toner image remain substantially unchanged. Therefore, the adjustment of the latent image write start timing or the adjustment of inclination of the optical components or both can reduce or prevent the positional displacement of dots and images between different colorants.
By contrast, misregistration caused by fluctuation in the speed of the belt member varies the relative positions of dots in each single-color toner image between colors, and therefore adjustments of the latent image write start timing and/or of inclination of the optical components cannot eliminate the misregistration.
When the speed fluctuation of the belt member occurs while the optical sensors are detecting pattern images for misregistration detection, the pattern images of different colors cannot be detected properly. Accordingly, the speed fluctuation of the belt member not only causes a color shift in a composite toner image but also prevents a correction of misregistration of respective single-color toner images induced by the relative displacement of the write start positions between the image carriers.
The above-described problems have also occurred in image forming apparatuses employing a multi-cycle electrophotographic printing scheme, in which a belt member rotates more than one time while transferring, per cycle, each visible image formed on an image carrier onto a surface of the belt member or a recording medium carried by the belt member.
Thus, there remains a need for improved transfer devices so as to suppress or eliminate a misregistration-induced color shift in a composite image due to relative displacements of write start positions between multiple image carriers and speed fluctuation of a belt member, and image forming apparatuses that include such a transfer device.